Since spectrum is a scarce resource and limited, and therefore, most of the current and emerging broadband wireless networks are expected to re-use the available spectrum in every sector/cell. Therefore, the performance of these types of systems will be limited by co-channel interference caused due to the frequency re-use mechanism. Especially in the existing IEEE 802.16e networks, the cell coverage is mainly limited by the downlink control channel (DL-MAP) coverage. It is determined by the ability of the users to correctly decode the DL-MAP, which carries essential control information such as downlink (DL) allocation and uplink (UL) grant, Multiple Input, Multiple Output (MIMO) schemes, redundancy version for Hybrid Automatic Request (HARQ), pilot format for data resource blocks, etc.
The area coverage probability in 16e type networks is defined as: P(BLERDL-Control-Channel≦BLERTarget), where BLER is the block error rate. The target BLER is typically chosen to be around 1%, and to achieve this lower error rate, the DL-MAP is modulated using Rate ½ QPSK (Quadrature Phase Shift Keying) and further repeated 2, 4, or 6 times. In a re-use-1 system deployment, a cell edge user typically receives at least 5 strong interferers (with a C/I of approximately −6 dB), and therefore, data must be repeated at least 6 times to meet the target BLER. Moreover, other link level enhancement techniques such cyclic delay diversity (CDD) and linear Minimum Mean Square Error (MMSE) interference suppression (using 2-antennas at the Mobile Station (MS)) may be required to meet the cell edge coverage requirement. For low frequency selectivity PED-A channel, CDD transmission provides a diversity advantage, and MMSE processing with two receive antennas at the MS provide additional interference cancellation (IC) gain. With 2-antennas, the IC gain will be somewhat limited since the MMSE receiver can cancel at most one co-channel interferer (CCI). Note that the receiver would be able to null all N-interferers only if the MS has at least (N+1) antennas. A significant gain in cell edge coverage can be obtained by suppressing all the CCI in interference limited networks.
Co-channel interference not only limits the control channel coverage, it also limits the spectrum efficiency/throughput of cell edge users. In both IEEE 802.16e and long-term-evolution (LTE) standards, users with low SINR are typically assigned a suitably chosen channel code rate together with simple bit or symbol level data repetition. In some cases, cell edge users are allocated a very low-rate channel code such that the cell edge user will be able to correctly decode its data. If the network has to maintain a certain quality of service (QoS) such as a sustained rate of 500 kbps for any user independent of the location in the cell, a large portion of the total available system bandwidth will be consumed by the cell edge users, which reduces the overall spectrum efficiency.
A contribution (C802.16m-07/211) by Panasonic in the IEEE 802.16m standards proposed that, instead of repeating the data in the DL-MAP at bit level one should repeat the QPSK modulation symbols “n” times and map these repeated data symbols to distinct subcarriers. If all base stations map their repeated data on the same set of subcarriers in a synchronous manner, then a receiver with “Nr” antennas can collect multiple copies of the signal along with interference from the “n” subcarriers of the interfering base stations to generate a total of n*Nr observations. With a repetition factor of n=4, and Nr=2 receive antennas, a total of n*Nr=8 observations can be obtained, and it can be used to potentially reject n*Nr−1=7 interferers.
A conventional maximum ratio combing (MRC) receiver provides a 3-dB SNR advantage with a repetition factor of two or in general, the SNR gain will be 10 Log(n) dB. The conventional receiver also provides an additional diversity gain when the subcarriers are sufficiently spaced apart in a frequency selective channel.
However, an MMSE receiver which jointly filters using MMSE weights will completely suppress the interference, if the subcarriers on which the data is repeated experience different channel gains. In other words, the channel should have high enough frequency selectivity. In Rician, line-of-sight, or flat fading channels, the channel will have limited or no variation across subcarriers resulting in incomplete suppression of the interference. It can be easily shown that the MMSE receiver will not be able to suppress the interference.
The techniques proposed in prior art requires the propagation channel on which the data is repeated to be distinct. In addition, this technique repeats the data tones sufficiently far apart in frequency. Therefore, the method proposed in prior art will not provide much advantage in Rician, line-of-sight, or flat fading channels. Moreover, if the data has to be repeated on distinct subcarriers which are spaced far apart in frequency, active co-ordinated transmission among different base station becomes difficult especially if the data payload in each BS is different. Because of these requirements, the prior art techniques is unsuitable for implementation in existing wireless networks such as IEEE 802.16m or LTE.